Cobra - or Cabinet Office Briefing Room A - so named because of the room its members meet in, coordinates the high-level response to serious incidents. It brings together government ministers with senior officials of the emergency services and security and intelligence agencies.

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A police officer stops traffic as the Jaguar car of British Prime Minister Theresa May is driven away from Parliament

A separate eyewitness who was in Parliament when the attack happened told HuffPost UK: “I heard what sounded like an explosion and raised voices outside Parliament, and so I rushed to the window.

“I saw people running past the entrance to the New Palace Yard entrance to Parliament, and then at least one person try to run into the Yard itself. A police officer chased this person and wrestled them to ground.

“Shots were then fired, but I can’t remember how many and I didn’t see who fired them.